Maximum drift is windspeed divided by airspeed in nautical miles per minute, or Max Drift = Windspeed x 60/Airspeed (knots).
Once you know the maximum drift then a rough rule of thumb calculation of actual drift is to apply a factor depending on the wind angle and using your watch face as a guide, e.g. if wind is 30 degrees off heading then the factor is 0.5 (half way around the watch face), if it is 15 degrees the factor is 0.25; anything over 60 degrees and the actual drift is the maximum drift! Quite easy to work out (you mark maximum drift somewhere on your chart before you go) and work it out actual drift in your head if you need to.
You will be unlikely in practice to fly to any greater degree of accuracy that a computer might give.
That is a recommendation that I cannot subscribe to. For long cross-country flights, a turning or increasing wind can mean the difference between making it with reserve and not making it at all. You want to anticipate this by simulating a variety of realistic wind strenghts and directions to see what gives.
Depends how long is "long" I suppose, but the longer the flight the less likely that the wind given pre departure is likely to be accurate, so you are going to have to recalculate on route anyway depending on actual wind. I agree that you would need to anticipate worse case by pre-calculating a maximum likely wind and its effect on range, but that is a different exercise from religiously making up your plog with data which has probably never been correct and then flying to it. I suspect anyone flying a distance far enough for that to be an issue is either going to have an aircraft with a huge range and a gps which will give fuel required to destination or they will be in something like a Cub which can land at a local filling station when the wind reduces the ground speed to 40 mph!!